Fused quartz capillaries 700 and 1200 m long and 100 μm in inside diameter have been filled under hydrostatic pressure. It is verified that the rate of filling is governed by Poiseuille's law, which predicts the filled length to be proportional to the square root of the filling time. At a pressure of 270 atm, 250 m was filled in 1 h, 800 m in 10 h, and 1200 m in 21 h. The filling rate cannot be increased significantly from the above values by increasing the pressure because of the increase in the viscosity of the fluid with increasing pressure. Since it requires high pressure to move the liquid in a long capillary, it is apparent that liquid will not flow out of a long capillary that is not under pressure.